Water management in proton exchange membrane (PEM) fuel cells has been a long standing problem. Proton exchange membrane fuel cells require certain humidity levels in their reactant streams to prevent dry-out of the electrolyte membrane or flooding of the cell, especially at the inlet and outlet of this device. These fuel cells must be able to maintain a high membrane ionic conductivity, or the cell performance will be seriously degraded. This necessitates that there be a high water content within the membrane. On the other hand, it is necessary that the build-up of liquid water not reach the stage whereby the cathode (typically air-handling) electrode and/or flow-field becomes flooded.
The present invention teaches how to alleviate flooding if a water-soluble fuel such as methanol is used to power the fuel cell. The current invention delivers reactant air via two or more flow channels in each cathode flow-field of a fuel cell stack, so that a flow channel in which air flows in a given direction is always adjacent to at least one, but generally two, flow channels, with air flowing in the opposite direction. This method allows moisture exchange between relatively wet and relatively dry adjacent cell regions. The amount of moisture exchange is, of course, dependent upon the respective moisture conditions of the air in the corresponding adjacent flow channels. The inlets and outlets of each fuel cell are maintained effectively adjacent to each other, as are the channels that connect to the inlets and outlets, respectively, to prevent the gases from becoming too water-laden. The channels of the fuel cell are designed with a serpentine (or other multi-pass) pattern, in one embodiment, so that a small number of adjacent inlet-outlet pairs can feed and collect reactant gases from the entire cell surface. Uniform artificial restrictions are provided at the inlets of each flow channel of every cell in order to obtain uniformity in air flow rate among all of the parallel flow channels throughout all cells of the fuel cell stack. This is accomplished by allowing the restrictions to dominate, and thereby render effectively uniform, the air flow resistances among the various flow channels.